Facilitation of acetylcholine release in rat frontal cortex by indeloxazine hydrochloride: involvement of endogenous serotonin and 5-HT4 receptors

Effects of indeloxazine hydrochloride, an inhibitor of serotonin (5-HT) and norepinephrine (NE) reuptake with a facilitatory effect on 5-HT release, on acetylcholine (ACh) output in frontal cortex of conscious rats were characterized using an in vivo microdialysis technique. Systemic administration of indeloxazine (3 and 10 mg/kg, i.p.) increased ACh and 5-HT output in a dose-dependent manner. Depletion of endogenous monoamines by reserpine and of 5-HT by p-chlorophenylalanine, but not that of catecholamines by α-methyl-p-tyrosine, significantly attenuated the facilitatory effect of indeloxazine on ACh release. When applied locally by reverse dialysis, indeloxazine (10 and 30 μM) and the selective 5-HT reuptake inhibitor citalopram (10 μM), but not the NE reuptake inhibitor maprotiline (30 μM), increased cortical ACh output. Indeloxazine (10 mg/kg)-induced increase in ACh release was significantly inhibited by local application of the 5-HT₄ receptor antagonists RS23597 (50 μM) and GR113803 (1 μM), while the 5-HT_1A antagonist WAY-100135 (100 μM), 5-HT_1A/1B/β-adrenoceptor antagonist (–)propranolol (150 μM), 5-HT_2A/2C antagonist ritanserin (10 μM) and 5-HT₃ antagonist ondansetron (10 μM) failed to significantly modify this effect. Neither depletion of monoamines nor treatment with serotonergic antagonists significantly changed the basal ACh level, indicating that endogenous monoamines do not tonically activate ACh release. These results suggest that indeloxazine-induced facilitation of ACh release in rat frontal cortex is mediated by endogenous 5-HT and involves at least in part cortical 5-HT₄ receptors. Received: 22 May 1997 / Accepted: 26 August 1997